Process for the non-oxidative preparation of formaldehyde from methanol

ABSTRACT

In a process for preparing formaldehyde from methanol by dehydrogenation in a reactor in the presence of a catalyst at a temperature in the range from 300 to 1000° C., a carrier gas stream which has a temperature above the dehydrogenation temperature is fed to the reactor.

This application is the US National Stage Application of PCT/EP98/03082filed May 26, 1998.

A number of processes for preparing formaldehyde from methanol are known(see, for example, Ullmann's Encyclopaedia of Industrial Chemistry). Theprocesses carried out industrially are predominantly the oxidation:

CH₃OH+½O₂→CH₂O+H₂O

over catalysts comprising iron oxide and molybdenum oxide at from 300°C. to 450° C. (Formox process) and the oxidative dehydrogenation (silvercatalyst process) according to:

CH₃OH→CH₂O+H₂

H₂+½O₂→H₂O

at from 600° C. to 720° C. In both processes, the formaldehyde is firstobtained as an aqueous solution. Particularly when used for thepreparation of formaldehyde polymers and oligomers, the formaldehydeobtained in this way has to be subjected to costly dewatering. A furtherdisadvantage is the formation of corrosive formic acid, which has anadverse effect on the polymerization, as by-product.

The dehydrogenation of methanol enables these disadvantages to beavoided and enables, in contrast to the abovementioned processes,virtually water-free formaldehyde to be obtained directly:

In order to achieve an ecologically and economically interestingindustrial process for the dehydrogenation of methanol, the followingprerequisites have to be met: the strongly endothermic reaction shouldbe carried out at high temperatures so that high conversions areachieved. Competing secondary reactions have to be suppressed in orderto achieve sufficient selectivity for formaldehyde (without catalysis,the selectivity for the formation of formaldehyde is less than 10% atconversions above 90%). The residence times have to be short or thecooling of the reaction products has to be rapid in order to minimizethe decomposition of the formaldehyde which is not thermodynamicallystable under the reaction conditions

CH₂O→CO+H₂.

Various methods of carrying out this reaction have been proposed; thus,for example, DE-A-37 19 055 describes a process for preparingformaldehyde from methanol by dehydrogenation in the presence of acatalyst at elevated temperature. The reaction is carried out in thepresence of a catalyst comprising at least one sodium compound at atemperature of from 300° C .to 800° C.

J. Sauer and G. Emig (Chem. Eng. Technol. 1995, 18, 284-291) were ableto set free a catalytically active species, which they presumed to besodium, from a catalyst comprising NaAlO₂ and LIAlO₂ by means of areducing gas mixture (87% N₂+13% H₂). The species was able to catalyzethe dehydrogenation of methanol introduced at a downstream point in thesame reactor, i.e. not coming into contact with the catalyst bed, togive formaldehyde. When using non-reducing gases, only a low catalyticactivity was observed.

According to J. Sauer and G. Emig and also results from more recentstudies (see, for example, M. Bender et al., paper presented to the 30thannual meeting of German catalyst technologists, Mar. 21-23, 1997),sodium atoms and NaO molecules were identified as species emitted intothe gas phase and their catalytic activity for the dehydrogenation ofmethanol in the gas phase was described. In the known processes, thestarting material methanol is always diluted with nitrogen and/ornitrogen/hydrogen mixtures for the reaction.

Although good results are achieved with the known processes, there isnevertheless considerable room for Improvement from a technical andeconomic point of view, particularly because the catalysts employedbecome exhausted or inactivated over time and the formaldehyde yieldsare still capable of improvement.

It has surprisingly been found that the yield in the dehydrogenation canbe increased if a carrier gas stream which has been brought to atemperature above the actual reaction temperature by heating isintroduced into the reactor. By means of such a superheated carrier gasstream, at least part of the heat required for the endothermicdehydrogenation reactor can be introduced.

An advantage here is that the heat of reaction does not have to betransferred to the gas stream via a hot wall, i.e. one having atemperature above the reaction temperature, in the reaction zone, butcan be introduced directly and more gently for the reaction gases bymeans of the separate heating and intensive mixing of the varioussubstreams. Decomposition of the unstable formaldehyde and secondaryreactions at the high temperatures in the reactor, in particular in thezones close to the wall, can thus be reduced.

The invention accordingly provides a process for preparing formaldehydefrom methanol by dehydrogenation in a reactor in the presence of acatalyst at a temperature in the range from 300 to 1000° C., wherein acarrier gas stream which has a temperature above the dehydrogenationtemperature is fed to the reactor.

The temperature difference between carrier gas stream anddehydrogenation temperature is preferably at least 20° C., particularlypreferably from 40 to 250° C.

The superheated gas stream can be fed directly into the reaction zone orall or part of it can be brought into contact with a primary catalyst(see below) beforehand.

The preferred temperatures for the superheated gas stream are from 600to 1000° C., particularly preferably from 700 to 900° C. Preferredtemperatures for the dehydrogenation of the methanol are from 500 to900° C.; particular preference is given to temperatures of from 600 to800° C.

The carrier gas stream or streams can consist of a reducing ornon-reducing gas, for example H₂/CO mixtures or nitrogen, preferably theby-products of the dehydrogenation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic overview of a preferred variant of the processof the invention.

The carder gas stream 1 is heated in the heat exchanger 2. Together withthe catalyst 4 coming from a reservoir 3, the total stream is introducedinto the reactor 5. Methanol 7 is conveyed from a reservoir 6, vaporizedin a heat exchanger 8 and likewise fed to the reactor 5. The productgases from the reactor 5 are cooled in the heat exchanger 9 and fed to aunit 10 for separating off the formaldehyde.

The invention further provides an apparatus for carrying out theabovementioned process comprising one or more heat exchangers forpreheating the starting materials, a vessel for superheating a carriergas stream, a heated reactor for carrying out the dehydrogenation, oneor more heat exchangers for cooling the product mixture, a unit forseparating off the formaldehyde and an apparatus for introduction of themethanol and for further introduction of a catalyst

For the purposes of the invention, dehydrogenation is a non-oxidativeprocess according to the equation:

Suitable catalysts are known, for example, from the literature, sees forexample, Chem, Eng. Technol. 1994, 17, 34.

Suitable metals are, for example, Li, Na, K, Cs, Mg, Al, In, Ga, Ag, Cu,Zn, Fe, Ni, Co, Mo, Ti, Pt or their compounds. Also suitable are, forexample, S, Se. phosphates of transition metals such as V and Fe, andheteropolyacids such as molybdophosphoric acid.

Examples of specific catalysts are:

sodium or sodium compounds (DE-A-37 19 055 and DE-A-38 11 509)

aluminum oxide, alkali metal aluminate and/or alkaline earth metalaluminate (EP-A04 05 348)

silver oxide (JP-A 60/089 441, Derwent Report 85 - 15 68 91126)

a catalyst comprising copper, zinc and sulfur (DE-A 25 25 174)

a catalyst comprising copper, zinc and selenium (U.S. Pat. No.4,054,609)

a catalyst comprising zinc and/or indium (EP-A 0 130 068)

silver (U.S. Pat. No. 2,953,602)

silver, copper and silicon (U.S. Pat. No. 2,939,883)

compounds containing zinc, cadmium, selenium, tellurium or indium.

Preference is given to using sodium or sodium compounds.

The form in which such a catalyst, for example a sodium-containingcatalyst, is used can vary widely: metallic, e.g. also as an alloy withat least one other alloy constituent, as compound or salt, where atleast one nonmetallic element is chemically combined with Na (binarycompounds and salts). If more than one element is present in chemicallycombined form in the compound, a binary, ternary or quaternary compoundor salt is present. Use of the catalyst in supported form, for exampleon an inorganic support, is likewise preferred.

If sodium is used in metallic form, it can be used as solid, liquid orpreferably as vapor. Preferred alloys are those with other alkali metalsand/or alkaline earth metals, e.g. Ba, Sr, Ca, Cs, Rb, K or particularlypreferably Li and/or magnesium.

Furthermore, alloys with B, Al, Si and Sn can also be used. This alsoapplies, in particular, to alloys which can comprise compounds such assodium boride NaB₂, sodium silicide NaSi or NaSn.

Examples of suitable binary sodium compounds and salts are sodiumcarbides such as Na₂C₂, NaC₈, sodium halides such as NaF, sodium oxidessuch as Na₂O, sodium azide, sodium phosphide, sodium sulfide, sodiumpolysulfides, preferably also sodium hydrides such as NaH.

Examples of suitable ternary sodium compounds and salts are sodiumborates such as borax, sodium phosphates or hydrogenphosphates, sodiumphosphites, sodium (meta)silicates and aluminosilicates, e.g. waterglass, Na₃AlF₆ (cryolite), sodium (hydrogen)sulfate, sodium sulfite,sodium nitrite, sodium nitrate, sodium amide, sodium acetylide NaCCH,sodium cyanide, sodium thiocyanate, the sodium salt of methyl thiol,sodium thiosulfate, but preferably NaOR where R═H or an organic radical(═salts of organic acids, alkoxides, phenoxides, acetylacetonate,acetoacetic ester salt, salts of salicylic acid or of salicylaldehyde),sodium carbonate and sodium hydrogencarbonate and mixtures thereof, forexample soda, thermonatrite, trona, pirssonite, natrocalcite. The use ofanhydrous, I.e. dried, salts is generally preferred. Particularpreference is given to NaOH, NaOOC-R⁻ (preferably formate, acetate,lactate, oxalate), NaOR′ (R′ is an organic radical having from 1 to 4carbon atoms) and sodium carbide. Very particular preference is given toNaOH, sodium formate, sodium methoxide, sodium acetate and sodiumcarbides such as Na₂C₂.

Examples of suitable quaternary compounds are sodium-containingaluminosilicates which can be prepared synthetically or can also occurin a wide variety as natural minerals and rocks (e.g. sodium feldspar orladen and calcium-sodium feldspar or oligoclase). They can additionallybe laden with Na by ion exchange.

Use can also advantageously be made of double salts of the alum type orthenardite, glauberite, astrakanite, glaserite, vanthoffite.

The sodium compounds and salts mentioned here can advantageously also bein the form of mixtures. In particular, it is quite possible to usecontents of <50%, preferably <30%, of cations of other alkali metalsand/or alkaline earth metals, e.g. Ba, Sr, Ca, Cs, Rb, K or preferablyLi and/or magnesium. Industrially available, complex mixtures such assoda lime, ground basic slag and cements, e.g. Portland cement, ifdesired after enrichment with sodium by storage in sodium-containingsolutions (NaCl, sea water) are particularly advantageous.

Particular preference is given to sodium compounds selected from thegroup consisting of:

a) sodium alkoxides,

b) sodium carboxylates,

c) sodium salts of C—H acid compounds,

d) sodium oxide, sodium hydroxide, sodium nitrite, sodium acetylide,sodium carbide, sodium hydride and sodium carbonyl.

The abovementioned catalysts will hereinafter be referred to as primarycatalyst.

In the process of the invention, the abovementioned compounds giveformaldehyde yields of over 60% and low water concentrations of lessthan 5 mol % of H₂O per mole of formaldehyde even at reactiontemperatures of from 600 to 1000° C.

The liberation of the catalytically active species from the primarycatalyst is preferably carded out by thermal decomposition of thelatter.

The primary catalyst can, for example, be introduced initially orafterwards, in each case continuously or discontinuously, as solid,dissolved in a solvent, as a liquid or as a melt.

The subsequent introduction of the primary catalyst as a solid, e.g. inpowder form, particulate or compacted, is generally carded out by meansof solids metering, e.g. using a reciprocating or rotary piston, acellular wheel feeder, a screw or a vibrating chute.

If the primary catalyst is added in dissolved form, particularlysuitable solvents are those having a chemical composition consisting ofonly the elements already present in the process (C, H, O). particularpreference is given to MeOH as solvent. The addition is carried out, forexample, via a noble which can be cooled in order to avoid evaporationof the solvent or crystallization or deposition of the solid primarycatalyst in the nozzle.

The addition of the primary catalyst as a melt can be carried out, forexample, via a nozzle. The melt can then be vaporized or decomposeddirectly in the gas stream.

For all possible ways of introducing further primary catalyst, this isadvantageously carded out in such a way that the material is in intimatecontact with flowing gas. This can be achieved, for example, by applyingthe catalyst material by the above-described methods onto a suitablesurface through or over which the gas flows. This can be the surface ofa support material which is present in a fixed beds Suitable materialsare, for example, SiC, SiO₂ and Al₂O₃ in a suitable geometric form, e.g.as granules, pellets or spheres. The support material is preferablyarranged vertically in a fixed bed, preferably with metering-in fromabove. The substance which is introduced deposits on the supportmaterial and the catalytically active species goes into the gas phaseduring the process.

Another possibility is placing the primary catalyst in a fluidized bedthrough which the carrier gas stream is passed. Here, the fluidizedmaterial comprises at least some of the supported or unsupported primarycatalyst. The loss of active substance can be made up by introducingfurther fresh primary catalyst; exhausted material can, if desired, betaken off. This can be realized in the continuous case, for example, bymeans of a circulating fluidized bed.

Further introduction of the primary catalyst can also be carried out byalternating secondary catalyst generation in different vessels in whichthe primary catalyst can be located, for example as a fixed bed or afluidized bed, in each case supported or unsupported. The advantage ofusing a plurality of units for the discontinuous introduction of furthercatalyst is that it is also possible to use primary catalysts for which,e.g. owing to material properties such as melting point, viscosity ordecomposition temperature, continuous feeding would be Impossible orpossible only with great difficulty.

In a preferred variant of the process of the invention, the secondarycatalyst is generated physically separately from the reaction zone inwhich the actual dehydrogenation takes place and at a temperature abovethe dehydrogenation temperature. The temperature difference between thesite of catalyst generation and the reaction zone is preferably at least20° C., particularly preferably from 40 to 250° C.

On thermal treatment of the primary catalysts according to the inventionin the primary catalyst decomposition zone and on passing a reducing ornon-reducing gas such as molecular nitrogen over them at temperatureswhich may be different from the reaction temperature for thedehydrogenation and may be higher or lower, one or more catalyticallyactive species which are able to catalyze the dehydrogenation ofmethanol are released or generated and/or generated on them (secondarycatalyst). Such a fluid catalyst can be transported over considerabledistances without suffering an appreciable loss of effectiveness in thedehydrogenation. This separate setting of temperatures makes itpossible, in particular, to lower the reaction temperature by matchingto the respective conditions for catalyst liberation/vaporization orgeneration of a catalytically active species (secondary catalysts on theone hand and to the reaction on the other hand. This reduces thedecomposition of the formaldehyde, which is unstable under the reactionconditions, as a result of secondary reactions and increases the yield.

Preferred temperatures for generating the secondary catalyst from theprimary catalyst are from 300 to 1100° C., particular preference isgiven to temperatures of from 400 to 1000° C.

In addition, the residence times in the dehydrogenation reactor andvessels for primary catalyst addition or for penetrating the secondarycatalyst can be set separately by dividing the carrier gas stream. Thisachieves a targeted loading of the gas stream passed through thecatalyst addition unit with the active species.

Preferred residence times for generating the secondary catalyst are from0.01 to 60 sec, particularly preferably from 0.05 to 3 sec.

Commercial methanol can be used for the reaction; it should preferablybe low in water and contain no substances which poison the catalyst.

To carry out the dehydrogenation, the fluid, preferably gaseous,methanol is preferably diluted with carrier gas.

The molar proportion of methanol is generally from 5 to 90%, preferablyfrom 10 to 50%, particularly preferably from 10 to 40%.

The pressure is not critical in the process of the invention. Thedehydrogenation of the methanol can be carried out at subatmosphericpressure, atmospheric pressure or subatmospheric pressure. A range fromabout 0.1 to 10 bars preferably from 0.5 to 2 bar, is particularlysuitable. Preference is given to atmospheric pressure. The process ofthe invention can be carried out discontinuously or continuously, withthe latter being preferred. The temperature is generally from 300° C. to950° C., preferably from 500 to 900° C., particularly preferably from600 to 850° C.

If the secondary catalyst is generated physically separately from thereaction zone, the temperatures in the reaction zone are generally from200 to 1000° C., preferably from 300° C. to 980° C. Preference is givento reacting from 0.01 to 1 kg of methanol per hour and per gram ofcatalyst used. In the case of a continuous process, further catalyst hasto be introduced continuously or discontinuously. The amounts here aregenerally from 10 milligrams to 5 grams, preferably from 10 mg to 1 g,particularly preferably from 50 to 1000 mg, very particularly from 50 mgto 500 mg. per kg of methanol reacted.

For the dehydrogenation of the methanol, residence times in the reactionzone are preferably from 0.005 to 30 sec, particularly preferably from0.01 to 15 sec, very particularly preferably from 0.05 to 3 sec.

Suitable reactors are well known to those skilled in the art.Essentially, it is possible to use reactor types and assemblies as areknown from the literature for dehydrogenation reactions. Suchapparatuses are described, for example, in Winnacker/Küchler, ChemischeTechnologie, 4th edition, chapter “Technik der Pyrolyse” Hanser Verlag,Munich 1981-86. Suitable reactors are, for example, tube reactors;suitable reactor materials are, for example, ceramic materials such asα-alumina but also iron- and nickel-based alloys which are resistant tocarbonization, heat and scale, e.g. Inconel 600® or Hasteloy®.

If the reactor 5 or the vessel 2 is heated by means of a combustionreaction, eternally fired tubes, for example, are suitable.

Preference is likewise given to heating the reactor by means ofmicrowaves.

In a further preferred variant of the process of the Invention, acirculating gas stream consisting essentially of by-products of thedehydrogenation is passed through the reactor,

Preference is also given to bleeding off part of the by-products fromthe circulating gas process and using this for firing the reactor.

The formaldehyde can be separated from the reaction mixture by methodsknown per se with which those skilled in the art are familiar, forexample by polymerization, condensation or physical or chemicalabsorption or adsorption.

An industrially proven method is the formation of hemiacetals fromformaldehyde and an alcohol. The hemiacetals are subsequentlydissociated thermally, giving very pure formaldehyde vapor. The alcoholused is usually cyclohexanol since its boiling point is sufficiently farabove the decomposition temperature of the hemiacetal. The hemiacetalsare usually dissociated in failing film or thin film evaporators attemperatures of from 100 to 160° C. (see, for example, U.S. Pat. No.2,848,500 of Aug. 19, 1958 “Preparation of Purified Formaldehyde” andU.S. Pat. No. 2,943,701 of Jul. 5, 1960 “Process for purification ofgaseous formaldehyde”, or JP-A 62/289 540). The formaldehyde vaporswhich are liberated in such a process still contain small amounts ofimpurities which are usually removed by means of a countercurrent scrubusing alcohol such as cyclohexanol hemiformal, by condensation or alsoby targeted prepolymerization.

Particularly preferred methods of purifying the formaldehyde preparedaccording to the invention are described in the German PatentApplications 19 747 647.3 and 19 748 380.1.

A further method of separating formaldehyde from the reaction mixture isthe formation of trioxane in a catalytic gas-phase process (see, forexample, Appl. Catalysis A 1997, 150, 143-151 and EP-A 0 691 338).Trioxane can then, for example, be condensed out.

Possible uses of the by-products of the reaction, in particularhydrogen, are, for example, the synthesis of methanol or the isolationof pure hydrogen which can be separated off, for example, by means ofmembranes.

Hydrogen obtained in this way is suitable, for example, for thesynthesis of ammonia, in refinery processes for producing gasoline andpetrochemical cracking products, for the synthesis of methanol, forhardening fats and for other hydrogenations, as reducing agent forproducing W, Mo, Co and other metals, as reducing protective gas inmetallurgical processes, for autogenous welding and cutting, as fuel gasin admixture with other gases (town gas, water gas) or in liquefied formas fuel in aerospace applications.

The formaldehyde prepared by the process of the invention is suitablefor all known fields of application, for example corrosion protection,production of mirrors, electrochemical coatings, for disinfection and asa preservative, likewise as an intermediate for producing polymers, forexample polyoxymethylenes, polyacetals, phenolic resins, melamines,aminoplastics, polyurethanes and casein plastics, and also 1,4-butanol,trimethylolpropane, neopentyl glycol, pentaerythritol and trioxane, formethanolic formaldehyde solutions and methylal, for producing dyes suchas fuchsin, acrydine, for producing fertilizers and for treating seed.

Since the process of the invention usually produces formaldehyde havinga low water content, formaldehyde prepared in this way is particularlysuitable for polymerization to give polyoxymethylene and trioxane, sincewater-free formaldehyde has to be used for this purpose.

The invention also relates to plastics such as polyoxymethylene andpolyacetals, trioxane, dyes, fertilizers and seed produced in such away.

The invention further provides a process for preparing trioxane, whichcomprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorat a temperature in the range from 300 to 1000° C. in the presence of acatalyst, where a carrier gas stream having a temperature above thedehydrogenation temperature is fed to the reactor, and

2. the formaldehyde prepared in this way is trimerized to give trioxane.

Details of the preparation of trioxane are well known to those skilledin the art. They are described, for example, in Kirk-Othmer,Encyclopedia of Chemical Technology, 2nd edition, volume 10, pp. 83, 89,New York Interscience 1963-1972.

The invention likewise provides a process for preparingpolyoxymethylene, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorat a temperature in the range from 300 to 1000° C. in the presence of acatalyst, where a carrier gas stream having a temperature above thedehydrogenation temperature is fed to the reactor, and

2. if desired, purifying the formaldehyde obtained in this way,

3. polymerizing the formaldehyde,

4. capping the end groups of the polymer prepared in this way and

5. if desired, homogenizing the polymer in the melt and/or providing itwith suitable additives.

The preparation of polyoxymethylene from formaldehyde is well known tothose skilled in the art. Details may be found, for example, inUllmann's Encyclopedia of Industrial Chemistry, volume 21, 5th edition,Weinheim 1992, and the literature cited therein.

The invention further provides a process for preparing polyoxymethylenecopolymers, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorat a temperature in the range from 300 to 1000° C. in the presence of acatalyst, where a carrier gas stream having a temperature above thedehydrogenation temperature is fed to the reactor, and

2. trimerizing the formaldehyde obtained in this way to give trioxane,

3. if desired, purifying the trioxane,

4. copolymerizing the trioxane with cyclic ethers or cyclic acetals,

5. if desired, removing unstable end groups and

6. if desired, homogenizing the polymer prepared in this way in the meltand/or admixing if with suitable additives.

The invention further provides a process for preparing polyoxymethylenecopolymers, which comprises

1. converting methanol into formaldehyde by dehydrogenation in a reactorin the presence of a catalyst at a temperature in the range from 300 to1000° C., where a circulating gas stream comprising by-products of thedehydrogenation is passed through the reactor, and

2. if desired, purifying the formaldehyde obtained in this way,

3. copolymerizing the formaldehyde with cyclic ethers or cyclic acetals,

4. if desired, removing unstable end groups and

5. if desired, homogenizing the polymer prepared in this way in the meltand/or admixing it with suitable additives.

The preparation of polyoxymethylene copolymers is well known to thoseskilled in the art. Details may be found, for example, in Ullmann'sEncyclopedia of Industrial Chemistry, volume 21, 5th edition, Weinheim1992 and the literature cited therein, and also in the Russian documentsSU 436067, 740715 and SU 72-1755156, 720303.

The contents of the priority-establishing German Patent. Applications197 22 774.0, 197 27 519.2 and 19743145.3 and also the Abstract of thepresent application are expressly incorporated by reference into thepresent description.

The invention is illustrated by the examples without being restrictedthereby.

EXAMPLES

FIG. 2 schematically shows the configuration of the experimentalapparatus by means of a flow diagram.

The dehydrogenation of the methanol is carried out in a tube reactor 26which is indirectly heated by means of an electric tube furnace 12. Acatalyst addition unit is formed by a metal tube 11 which is indirectlyheated by the electric tube furnace 12. In the tube 11, there is a bed13 of support material on which the primary catalyst (0.1-5.0 g) islocated. A part 14 of a superheated carrier gas stream 15 which has beenpreheated beforehand by means of heated feed lines is introduced intothis tube 11. In addition, further primary catalyst is fed as a solutionvia a nozzle 16 into this tube 11. The primary catalyst deposits on thebed 13. The carrier gas substream 14 is passed through the bed in orderto load the carrier gas substream with an active catalyst species whichforms. The total stream is subsequently introduced into the reactionspace 19.

Methanol 17 is preheated, conveyed in a further part 18 of the carriergas stream 15 and likewise introduced into the reaction space 19.

A third gas stream 20 consisting of pure carrier gas 15 is superheated21, i.e. brought to a temperature which is above the dehydrogenationtemperature, and likewise introduced into the reaction space 19.

The reaction space 19 is formed by a tube having a length of 200-450 mm,internal diameter 4-21 mm. In a cooler 22, the product gases leaving thereaction space 19 are quickly cooled to a temperature below 200° C. andare analyzed by means of a gas chromatograph. In a column 23, thereaction products are scrubbed with alcohol 24 (e.g. cyclohexanol at20-80° C.) in order to remove the formaldehyde 25. The primary catalystused is sodium methoxide, the carrier gas used is H₂/CO or nitrogen. Thetotal flow is 20-500 l/h, at least 50% of the carrier gas stream is feddirectly to the reactor after superheating. The methanol feed rate issuch that a methanol concentration of about 520 mol % is established.

The formaldehyde yield is calculated as follows.${{Yield}\quad \left( {{in}\quad \%} \right)} = {\frac{{formaldehyde}\quad {formed}\quad ({mol})}{{methanol}\quad {fed}\quad {in}\quad ({mol})} \cdot 100}$

Furnace Furnace Example/ temperature Temperature temperature for Yieldof Comparative for catalyst of carrier gas reactor, formal- Exampledecomposition stream dehydrogenation dehyde Example 1 900° C. 870° C.750° C. 76% Example 2 880° C. 870° C. 750° C. 74% CE 1 900° C. 820° C.750° C. 72%

What is claimed is:
 1. A process for preparing formaldehyde frommethanol by dehydrogenation in a reactor in the presence of a catalystat a temperature in the range from 300 to 1000° C., wherein a carriergas stream which has a temperature above the dehydrogenation temperatureis fed to the reactor.
 2. The process as claimed in claim 1, wherein thetemperature difference is at least 20° C.
 3. The process as claimed inclaim 1, wherein the catalyst is generated physically separately fromthe reactor.
 4. The process as claimed in claim 1, wherein a primarycatalyst is used for generating the catalyst and further amounts of thisprimary catalyst are introduced continuously.
 5. The process as claimedin claim 1, wherein the carrier gas steam consists essentially of theby-products of the dehydrogenation.
 6. The process as claimed in claim1, wherein part of the by-products of the dehydrogenation is used asfuel for heating the reactor.
 7. The process as claimed in claim 1,wherein the hydrogen formed as by-product is separated off.
 8. Theprocess as claimed in claim 1, wherein the catalyst used is one or moresodium compounds which are selected from the group consisting of: sodiumalkoxides, sodium carboxylates, sodium salts of C—H acid compounds,sodium oxide, sodium hydroxide, sodium nitrite, sodium acetylide, sodiumcarbide, sodium hydride and sodium carbonyl.
 9. A process as claimed inclaim 1 for preparing formaldehyde from methanol by dehydrogenation ofthe methanol at a temperature in the range from 300 to 1000° C. in thepresence of a catalytically active species set free from a sodiumcompound, wherein the sodium compound used is: a) sodium alkoxides, b)sodium carboxylates, c) sodium salts of C—H acid compounds or d) sodiumoxide, sodium hydroxide, sodium nitrite, sodium acetylide, sodiumcarbide, sodium hydride or sodium carbonyl.
 10. The process as claimedin claim 9, wherein the sodium compound used is a) sodium alkoxides oflower (C₁-C₆)alcohols or glycolates; b) sodium carboxylates of lower(C₁-C₆)monocarboxylic, dicarboxylic, tricarboxylic or hydroxylcarboxylicacids; c) sodium acetylacetonate or sodium salt of ethyl acetoacetate ord) sodium carbide.
 11. The process as claimed in claim 10, wherein thesodium compound used is a) NaOCH₃, NaOC₂H₅ or NaO—(CH₂)₂—ONa or b)NaOOCH, NaOOC—CH₃, NaOOC—C₂H₅ or NaOOC—CH(OH)—CH₃; c) Na₂C₂.
 12. Theprocess as claimed in claim 11, wherein the sodium compound used isNaOCH₃, NaOOCH, NaOOC—CH₃ and/or Na₂C₂.
 13. The process as claimed inclaim 9, wherein the sodium compound is fed in, or further amounts arefed in, as a solid, as a solution in a solvent, as a liquid or as amelt, in each case continuously or discontinuously.
 14. The process asclaimed in claim 13, wherein the dehydrogenation is carried out in areactor and the generation of the catalytically active species iscarried out physically separately from the reactor.
 15. The process asclaimed in claim 9, wherein the dehydrogenation is carried out in areactor and a circulating gas steam which consists essentially ofby-products from the dehydrogenation is passed through the reactor. 16.The process as claimed in claim 9, wherein the dehydrogenation iscarried out in a reactor and part of the by-products of thedehydrogenation are used as fuel for heating the reactor.
 17. Theprocess as claimed in claim 9, wherein the methanol is diluted with acarrier gas and the carrier gas stream is heated before it is combinedwith the methanol.
 18. The process as claimed in claim 9, wherein thehydrogen obtained as by-product is separated off and passed on to afurther use.